A series of mononuclear nickel(II) bis(diphosphine) complexes [Ni(P(Ph)(2)N(C6H4X)(2))(2)](BF(4))(2) (P(Ph)(2)N(C6H4X)(2) = 1,5-di(para-X-phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; X = OMe, Me, CH(2)P(O)(OEt)(2), Br, and CF(3)) have been synthesized and characterized. X-ray diffraction studies reveal that [Ni(P(Ph)(2)N(C6H4Me)(2))(2)](BF(4))(2) and [Ni(P(Ph)(2)N(C6H4OMe)(2))(2)](BF(4))(2) are tetracoordinate with distorted square planar geometries. The Ni(II/I) and Ni(I/0) redox couples of each complex are electrochemically reversible in acetonitrile with potentials that are increasingly cathodic as the electron-donating character of X is increased. Each of these complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni(II/I) couple. The catalytic rates generally increase as the electron-donating character of X is decreased, and this electronic effect results in the favorable but unusual situation of obtaining higher catalytic rates as overpotentials are decreased. Catalytic studies using acids with a range of pK(a) values reveal that turnover frequencies do not correlate with substrate acid pK(a) values but are highly dependent on the acid structure, with this effect being related to substrate size. Addition of water is shown to dramatically increase catalytic rates for all catalysts. With [Ni(P(Ph)(2)N(C6H4CH2P(O)(OEt)2)(2))(2)](BF(4))(2) using [(DMF)H](+)OTf(-) as the acid and with added water, a turnover frequency of 1850 s(-1) was obtained.

Transgenic tobacco plants expressing either a full-length form of the tobacco etch virus (TEV) coat protein or a form truncated at the N terminus of the TEV coat protein were initially susceptible to TEV infection, and typical systemic symptoms developed. However, 3 to 5 weeks after a TEV infection was established, transgenic plants "recovered" from the TEV infection, and new stem and leaf tissue emerged symptom and virus free. A TEV-resistant state was induced in the recovered tissue. The resistance was virus specific. Recovered plant tissue could not be infected with TEV, but was susceptible to the closely related virus, potato virus Y. The resistance phenotype was functional at the single-cell level because protoplasts from recovered transgenic tissue did not support TEV replication. Surprisingly, steady state transgene mRNA levels in recovered tissue were 12-to 22-fold less than transgene mRNA levels in uninoculated transgenic tissue of the same developmental stage. However, nuclear run-off studies suggested that transgene transcription rates in recovered and uninoculated plants were similar. We propose that the resistant state and reduced steady state levels of transgene transcript accumulation are mediated at the cellular level by a cytoplasmic activity that targets specific RNA sequences for inactivation.

Tobacco etch potyvirus engineered to express the reporter protein beta‐glucuronidase (TEV‐GUS) was used for direct observation and quantitation of virus translocation in plants. Four TEV‐GUS mutants were generated containing capsid proteins (CPs) with single amino acid substitutions (R154D and D198R), a double substitution (DR), or a deletion of part of the N‐terminal domain (delta N). Each modified virus replicated as well as the parental virus in protoplasts, but was defective in cell‐to‐cell movement through inoculated leaves. The R154D, D198R and DR mutants were restricted essentially to single, initially infected cells. The delta N variant exhibited slow cell‐to‐cell movement in inoculated leaves, but was unable to move systemically due to a lack of entry into or replication in vascular‐associated cells. Both cell‐to‐cell and systemic movement defects of each mutant were rescued in transgenic plants expressing wild‐type TEV CP. Cell‐to‐cell movement, but not systemic movement, of the DR mutant was rescued partially in transgenic plants expressing TEV CP lacking the C‐terminal domain, and in plants expressing CP from the heterologous potyvirus, potato virus Y. Despite comparable levels of accumulation of parental virus and each mutant in symptomatic tissue of TEV CP‐expressing transgenic plants, virions were detected only in parental virus‐ and delta N mutant‐infected plants, as revealed using three independent assays. These data suggest that the potyvirus CP possesses distinct, separable activities required for virion assembly, cell‐to‐cell movement and long‐distance transport.

No abstract

Transgenic tobacco plants expressing either a full-length form of the tobacco etch virus (TEV) coat protein or a form truncated at the N terminus of the TEV coat protein were initially susceptible to TEV infection, and typlcal systemic symptoms developed. However, 3 to 5 weeks after a TEV infection was established, transgenlc plants "recovered" from the TEV infection, and new stem and leaf tissue emerged symptom and virus free. A TEV-resistant state was induced ln the recovered tissue. The resistance was virus speclflc. Recovered plant tissue could not be lnfected wlth TEV, but was susceptlble to the closely related virus, potato virus Y. The resistance phenotype was functional at the slngle-cell leve1 because protoplasts from recovered transgenic tissue did not support TEV repllcation. Surprlslngly, steady state transgene mRNA levels in recovered tissue were 12-to 22-fold less than transgene mRNA levels in uninoculated transgenic tissue of the same developmental stage. However, nuclear run-off studles suggested that transgene transcription rates in recovered and uninoculated plants were similar. We propose that the resistant state and reduced steady state levels of transgene transcript accumulation are mediated at the cellular leve1 by a cytoplasmic actlvity that targets speclfic RNA sequences for inactlvation.